EcoSal Plus最新文献

Two-component regulatory systems represent the major paradigm for signal transduction in prokaryotes. The simplest systems are composed of a sensor kinase and a response regulator. The sensor is often a membrane protein that senses a change in environmental conditions and is autophosphorylated by ATP on a histidine residue. The phosphoryl group is transferred onto an aspartate of the response regulator, which activates the regulator and alters its output, usually resulting in a change in gene expression. In this review, we present a historical view of the archetype EnvZ/OmpR two-component signaling system, and then we provide a new view of signaling based on our recent experiments. EnvZ responds to cytoplasmic signals that arise from changes in the extracellular milieu, and OmpR acts canonically (requiring phosphorylation) to regulate the porin genes and noncanonically (without phosphorylation) to activate the acid stress response. Herein, we describe how insights gleaned from stimulus recognition and response in EnvZ are relevant to nearly all sensor kinases and response regulators.

Phosphorus is required for many biological molecules and essential functions, including DNA replication, transcription of RNA, protein translation, posttranslational modifications, and numerous facets of metabolism. In order to maintain the proper level of phosphate for these processes, many bacteria adapt to changes in environmental phosphate levels. The mechanisms for sensing phosphate levels and adapting to changes have been extensively studied for multiple organisms. The phosphate response of Escherichia coli alters the expression of numerous genes, many of which are involved in the acquisition and scavenging of phosphate more efficiently. This review shares findings on the mechanisms by which E. coli cells sense and respond to changes in environmental inorganic phosphate concentrations by reviewing the genes and proteins that regulate this response. The PhoR/PhoB two-component signal transduction system is central to this process and works in association with the high-affinity phosphate transporter encoded by the pstSCAB genes and the PhoU protein. Multiple models to explain how this process is regulated are discussed.

Plasmids are ubiquitous in the microbial world and have been identified in almost all species of bacteria that have been examined. Their localization inside the bacterial cell has been examined for about two decades; typically, they are not randomly distributed, and their positioning depends on copy number and their mode of segregation. Low-copy-number plasmids promote their own stable inheritance in their bacterial hosts by encoding active partition systems, which ensure that copies are positioned in both halves of a dividing cell. High-copy plasmids rely on passive diffusion of some copies, but many remain clustered together in the nucleoid-free regions of the cell. Here we review plasmid localization and partition (Par) systems, with particular emphasis on plasmids from Enterobacteriaceae and on recent results describing the in vivo localization properties and molecular mechanisms of each system. Partition systems also cause plasmid incompatibility such that distinct plasmids (with different replicons) with the same Par system cannot be stably maintained in the same cells. We discuss how partition-mediated incompatibility is a consequence of the partition mechanism.

The Tat pathway for protein translocation across bacterial membranes stands out for its selective handling of fully folded cargo proteins. In this review, we provide a comprehensive summary of our current understanding of the different known Tat components, their assembly into different complexes, and their specific roles in the protein translocation process. In particular, this overview focuses on the Gram-negative bacterium Escherichia coli and the Gram-positive bacterium Bacillus subtilis. Using these organisms as examples, we discuss structural features of Tat complexes alongside mechanistic models that allow for the Tat pathway's unique protein proofreading and transport capabilities. Finally, we highlight recent advances in exploiting the Tat pathway for biotechnological benefit, the production of high-value pharmaceutical proteins.

Like all outer membrane (OM) constituents, integral OM β-barrel proteins in Gram-negative bacteria are synthesized in the cytoplasm and trafficked to the OM, where they are locally assembled into the growing OM by the ubiquitous β-barrel assembly machine (Bam). While the identities and structures of all essential and accessory Bam components have been determined, the basic mechanism of Bam-assisted OM protein integration remains elusive. Here we review mechanistic analyses of OM β-barrel protein folding and Bam dynamics and summarize recent insights that inform a general model for OM protein recognition and assembly by the Bam complex.

The chaperone-usher (CU) pathway is a conserved secretion system dedicated to the assembly of a superfamily of virulence-associated surface structures by a wide range of Gram-negative bacteria. Pilus biogenesis by the CU pathway requires two specialized assembly components: a dedicated periplasmic chaperone and an integral outer membrane assembly and secretion platform termed the usher. The CU pathway assembles a variety of surface fibers, ranging from thin, flexible filaments to rigid, rod-like organelles. Pili typically act as adhesins and function as virulence factors that mediate contact with host cells and colonization of host tissues. Pilus-mediated adhesion is critical for early stages of infection, allowing bacteria to establish a foothold within the host. Pili are also involved in modulation of host cell signaling pathways, bacterial invasion into host cells, and biofilm formation. Pili are critical for initiating and sustaining infection and thus represent attractive targets for the development of antivirulence therapeutics. Such therapeutics offer a promising alternative to broad-spectrum antibiotics and provide a means to combat antibiotic resistance and treat infection while preserving the beneficial microbiota. A number of strategies have been taken to develop antipilus therapeutics, including vaccines against pilus proteins, competitive inhibitors of pilus-mediated adhesion, and small molecules that disrupt pilus biogenesis. Here we provide an overview of the function and assembly of CU pili and describe current efforts aimed at interfering with these critical virulence structures.

In 1989, Normark and coworkers reported on fibrous surface structures called curli on strains of Escherichia coli that were suspected of causing bovine mastitis. Subsequent work by many groups has revealed an elegant and highly regulated curli biogenesis pathway also referred to as the type VIII secretion system. Curli biogenesis is governed by two divergently transcribed operons, csgBAC and csgDEFG. The csgBAC operon encodes the structural subunits of curli, CsgA and CsgB, along with a chaperone-like protein, CsgC. The csgDEFG operon encodes the accessory proteins required for efficient transcription, secretion, and assembly of the curli fiber. CsgA and CsgB are secreted as largely unstructured proteins and transition to β-rich structures that aggregate into regular fibers at the cell surface. Since both of these proteins have been shown to be amyloidogenic in nature, the correct spatiotemporal synthesis of the curli fiber is of paramount importance for proper functioning and viability. Gram-negative bacteria have evolved an elegant machinery for the safe handling, secretion, and extracellular assembly of these amyloidogenic proteins.

Type III protein secretion systems (T3SSs), or injectisomes, are multiprotein nanomachines present in many Gram-negative bacteria that have a sustained long-standing close relationship with a eukaryotic host. These secretion systems have evolved to modulate host cellular functions through the activity of the effector proteins they deliver. To reach their destination, T3SS effectors must cross the multibarrier bacterial envelope and the eukaryotic cell membrane. Passage through the bacterial envelope is mediated by the needle complex, a central component of T3SSs that expands both the inner and outer membranes of Gram-negative bacteria. A set of T3SS secreted proteins, known as translocators, form a channel in the eukaryotic plasma membrane through which the effector proteins are delivered to reach the host cell cytosol. While the effector proteins are tailored to the specific lifestyle of the bacterium that encodes them, the injectisome is conserved among the different T3SSs. The central role of T3SSs in pathogenesis and their high degree of conservation make them a desirable target for the development of antimicrobial therapies against several important bacterial pathogens.

Lipoproteins are produced by both Gram-positive and Gram-negative bacteria. Once secreted, lipoproteins are quickly acylated, anchoring them into the plasma membrane. Recent work has shown that Gram-positive bacteria are able to generate considerable diversity in the acylation of their lipoproteins, though the mechanisms involved are only just beginning to emerge. In Gram-negative organisms, most lipoproteins are subsequently trafficked to the outer membrane (OM). Lipoprotein trafficking is an essential pathway in these bacteria. At least one OM lipoprotein component is required by each of the essential machines that assemble the OM (such as the Bam and Lpt machines) and build the peptidoglycan cell wall (Lpo-penicillin-binding protein complexes). The Lol pathway has been the paradigm for OM lipoprotein trafficking: a complex of LolCDE extracts lipoproteins from the plasma membrane, LolA shuttles them through the periplasmic space, and LolB anchors them into the OM. The peptide signals responsible for OM-targeting via LolCDE have long been known for Escherichia coli. Remarkably, production of novel lipoprotein acyl forms in E. coli has reinforced the idea that lipid signals also contribute to OM targeting via LolCDE. Moreover, recent work has shown that lipoprotein trafficking can occur in E. coli without either LolA or LolB. Therefore, current evidence suggests that at least one additional, LolAB-independent route for OM lipoprotein trafficking exists. This chapter reviews the posttranslocation modifications of all lipoproteins, with a focus on the trafficking of lipoproteins to the OM of Gram-negative bacteria.

The formation of disulfide bonds is critical to the folding of many extracytoplasmic proteins in all domains of life. With the discovery in the early 1990s that disulfide bond formation is catalyzed by enzymes, the field of oxidative folding of proteins was born. Escherichia coli played a central role as a model organism for the elucidation of the disulfide bond-forming machinery. Since then, many of the enzymatic players and their mechanisms of forming, breaking, and shuffling disulfide bonds have become understood in greater detail. This article summarizes the discoveries of the past 3 decades, focusing on disulfide bond formation in the periplasm of the model prokaryotic host E. coli.